1. Field of the Invention
The present invention relates, in general, to multistrand post-tensioning cable systems for use in fabricating a wide variety of concrete structures, such as bridges, parking garages, buildings and the like and in particular to a novel multistrand anchor head assembly for use in such systems.
2. Description of the Related Art
A typical multistrand post-tensioning cable system used in the fabrication of various concrete structures, such as those described above, comprises a bearing plate and an anchor head. The bearing plate comprises either a rectangular or a cylindrical member having a rear or bearing surface for bearing against a concrete structure. The bearing plate has a bore centrally located therein having an interior end and an exterior end. The interior end of the bore is provided for receiving a plurality of strands or tendons captured in and extending from corresponding strand or tendon anchoring apparatus in a remote concrete structure. The exterior end of the bore is provided for receiving an anchor head which is provided with means for capturing the ends of the strands or tendons received in the interior end of the bore.
The typical anchor head comprises either a rectangular or a cylindrical member having a rear or bearing surface which is inserted in and/or bears against the bearing plate. It is provided with a plurality of strand or tendon receiving cavities for receiving and capturing, with the help of wedge-shaped members, a corresponding number of strands or tendons.
The conventional anchor head typically is made from a relatively massive and heavy casting or forging of ductile iron or mild steel sufficient to withstand the tremendous compressive forces imparted to it via the strands or tendons captured therein. Its front and rear surfaces are generally planar and no attempt has been made heretofore to remove excessive material therefrom or to provide any sort of self-centering capability.
A typical strand receiving cavity in the conventional anchor head comprises a first or interior cylindrical cavity and a second or exterior cavity having the shape of a frustum of a cone in which a strand, after passing through the first cavity, is captured using two or more wedge-shape members.
In addition to the weight and massiveness of the prior known anchor head and the lack of any self-centering features therein, a further disadvantage of the prior known anchor head is that the bundle of strands in the cable captured therein must be splayed at the anchor head and that the cylindrical shape of the first cavity in the conventional anchor head tends to impart bending moments to the strands which increase in magnitude as a function of the distance a strand is from the center of the bundle. These bending moments are found to impart an undesirably large amount of stress and fatigue to the strands in the bundle, particularly in the outermost strands therein.